Objectives of the proposed research center primarily on assessment of the influence of genetic differences on continuously varying characters associated with thermoregulation, with emphasis on behavioral phenotypes. Although most characters of importance to animals, including humans, belong in this category, there has been a paucity of genetic analyses in biological investigations of quantitatively varying phenotypes. Specific aims of this research include examination of the extent of common genetic bases among the traits under investigation, as well as the contribution of genotype by temperature interactions to differences in expression of these characters. The primary genetic analysis will involve bidirectional selection (replicated with control lines) for nest-building in laboratory mice. Genetic correlations for activity, thermal preference, and metabolic heat production (as well as body weight and food consumption) will be estimated from their correlated responses to selection for nesting. Genotype-environment interactions will be examined by testing across selected lines at warm and cold temperatures, using a split-litter design.